1. Field of the Invention
The present invention relates generally to a spacecraft attitude sensor, and more specifically to such a sensor which is equipped with an optical apparatus for effectively expanding the usage of the sensor by suitably deviating light rays entering the sensor.
2. Description of the Prior Art
Before turning to the present invention it is deemed advantageous to discuss a known spacecraft attitude sensor with reference to FIGS. 1-4.
Throughout the instant specification, an attitude sensor is discussed for a satellite in a geostationary orbit merely for the convenience of descriptions.
FIG. 1 is a schematic diagram showing a known attitude sensor 10 which is mounted on a three-axis stabilized satellite and which is arranged to detect the Earth 12 for satellite attitude determination. Such an attitude sensor is disclosed in articles: (a) entitled "Development of Precision Earth Sensor for ETS-VI Satellite" by Fumiho TAKAHASHI, et al., NEC Res. & Develop., No. 97, April 1990, Pages 35-42 and (b) entitled "Pyroelectric Infrared Detector for Precision Earth Sensor" by Kunio NAKAMURA, et al., International Journal of Infrared and Millimeter Waves, Vol. 10, No. 8, 1989.
As shown in FIG. 1, the spacecraft attitude sensor 10 is comprised of a scan mirror 14, a lens 16, and a pair of infrared detectors 18a, 18b. It is assumed that the satellite which is equipped with the sensor 10 is at an altitude of 35,786 km where the period of the satellite equals the sidereal rotation period of the Earth 12. The scan mirror 14 oscillates or reciprocates at 8 Hz about a pitch axis 19 within the range defined by from .+-.12.degree. to .+-.15.degree.. The detectors 18a, 18b are canted by .+-.6.07.degree. (or .+-.3.82.degree.) relative to the equator of the Earth 12.
It is assumed that the satellite is correctly orientated and thus at zero attitude angle. When the scan mirror 14 oscillates, the detectors 18a and 18b detect 15 .mu.m infrared (CO.sub.2 absorption band) at 45.degree. north/south latitudes with the canted angles of .+-.6.07.degree.. (If the canted angles are .+-.3.82.degree. then the detectors 18a, 18b detect same band infrared at 27.degree. north/south latitudes.)
The detectors 18a, 18b define two "field of view" infrared beam channels, respectively. Each of two small circles 20a, 20b is called an instantaneous field of view (IFOV) which is determined by the shape of a received light spot and the optical characteristics of the lens 16. The horizontal scans in parallel with given north/south latitudes are respectively called "north side scan" and "south side scan". When the mirror 14 swings the field of view light beam channels, the IFOVs 20a, 20b move along the lines of the north/south side scans.
Although not shown in FIG. 1, the outputs of the infrared detectors 18a, 18b are applied to suitable electronics circuits and undergo digital signal processing.
Roll/pitch angles, which indicate satellite attitude errors, will be described with reference to FIG. 2 wherein two output pulses derived from the pair of detectors 18a, 18b are shown. Other than this, FIG. 2 is substantially identical with FIG. 1.
When the scan mirror 14 oscillates so as to swing the field of view infrared beam channels, the IFOVs 20a, 20b reciprocate along the north/south side scans. Thus, the detectors 18a, 18b detect the limbs of the Earth 12 and then generate electrical pluses 22a, 22b whose pulse widths are denoted by CWn, CWs, respectively. If the pitch axis 19, which passes through the center of the scan mirror 14, is selected as a reference pitch axis of the satellite, then the distances of the two limb detecting points from the axis 19 can be measured which are denoted by A and B in FIG. 2. Thus, roll and pitch angles relative to the center of the Earth 12 are given by EQU Roll angle=Cr(CWs-CWn) (1) EQU Pitch angle=Cp(A-B) (2)
where Cr, Cp: constants previously determined.
Equations (1) and (2) are well known to those skilled in the art and hence further discussions thereof will be omitted for the sake of brevity.
The sensor 10 is designed for determining the satellite attitude at an altitude of about 36,000 km. Accordingly, the sensor 10 is rendered unavailable if the satellite is at a very high altitude of about 85,000 km (for example) or at a very low altitude of about 1,000 km (for example). These problems come from the fact that: (a) the canted angle is fixed to .+-.6.07.degree. (or .+-.3.82.degree.) or (b) the mirror oscillating range can not be changed.
In more specific terms, if a satellite is located at a very high altitude of 85,000 km, the angle of view field of the Earth is as small as about 4.degree.. In this case, as illustrated in FIG. 3, the north and south side scans fall outside the disk of the Earth 12 at the zero attitude of the satellite. This indicates that the roll angle of the satellite are no longer determined.
Contrarily, if a satellite is located at a very low altitude of 987 km, the angle of view field of the Earth becomes as large as about 60.degree.. In this instance, as illustrated in FIG. 4, neither of the north/south side scans crosses the limbs of the Earth at the zero attitude. This indicates that the roll/pitch angles of the satellite can not be detected.
As mentioned above, the applications of the known sensor is strictly limited and hence, it is highly desirable to effectively increase the usage of the sensor in view of the high manufacturing cost thereof (for example).